The US military says the expected release of classified documents on the Wikileaks website could endanger US and allied troops and Iraqi civilians.
The whistleblower website is thought to be about to post hundreds of thousands of US military files on the Iraq war.
Nato chief Anders Fogh Rasmussen said the leak could have "very negative security implications".
A US defence spokesman said the documents concerned "significant activities" reported by units.
The Pentagon has assembled a team of more than 100 analysts to prepare for the release of the documents, which was expected later on Friday.
'Snapshots'A US defence department spokesman said the "stolen" documents should be returned to avoid potentially damaging information being released.
"By disclosing such sensitive information, Wikileaks continues to put at risk the lives of our troops, their coalition partners and those Iraqis and Afghans working with us," said spokesman Geoff Morrell.
He said the documents were "essentially snapshots of events, both tragic and mundane, and do not tell the whole story.
"That said, the period covered by these reports has been well-chronicled in news stories, books and films and the release of these field reports does not bring new understanding to Iraq's past".
He added: "However, it does expose secret information that could make our troops even more vulnerable to attack in the future".
BBC News
